// from https://github.com/influxdata/influxdb/tree/master/pkg/mmap

package gmmap

import (
    "os"
    "syscall"
    "unsafe"
)

// Map memory-maps a file.
func Map(path string) ([]byte, error) {
    f, err := os.Open(path)
    if err != nil {
        return nil, err
    }
    defer f.Close()

    fi, err := f.Stat()
    if err != nil {
        return nil, err
    } else if fi.Size() == 0 {
        return nil, nil
    }

    lo, hi := uint32(fi.Size()), uint32(fi.Size()>>32)
    fmap, err := syscall.CreateFileMapping(syscall.Handle(f.Fd()), nil, syscall.PAGE_READONLY, hi, lo, nil)
    if err != nil {
        return nil, err
    }
    defer syscall.CloseHandle(fmap)

    ptr, err := syscall.MapViewOfFile(fmap, syscall.FILE_MAP_READ, 0, 0, uintptr(fi.Size()))
    if err != nil {
        return nil, err
    }
    data := (*[1 << 30]byte)(unsafe.Pointer(ptr))[:fi.Size()]

    return data, nil
}

// Unmap closes the memory-map.
func Unmap(data []byte) error {
    if data == nil {
        return nil
    }
    return syscall.UnmapViewOfFile(uintptr(unsafe.Pointer(&data[0])))
}
